When operating micromirrors, including piezoelectric micromirrors operated in resonance, a frequent objective is to implement both a high resonant frequency and a large deflection of the micromirror. However, it is difficult to achieve a high resonant frequency in combination with a large deflection of the micromirror at the same time. U.S. Pat. No. 7,190,502 B2 describes a device using which a deflection of 12.4 mm can be achieved at a resonant frequency of 10.6 kHz. U.S. Pat. No. 8,125,699 B2 shows devices using which an amplitude of 5.25 mm can be achieved at a resonance frequency of 15.6 kHz and 9 mm at a resonant frequency of 304 Hz. Devices of U.S. Pat. No. 6,657,764 B1 can be operated at amplitudes of 40 mm at a resonant frequency of 500 Hz and an amplitude of 3.9 mm at a resonant frequency of 17.2 kHz.
In order to simultaneously achieve large deflections and high resonant frequencies, EP 2233 961 A1 discloses a setup in which a vibratable, oscillating system comprises laterally arranged actuators and a micromirror arranged in the center which, connected to one another via a torsion spring, form a vibratable overall system and exhibit a common resonant frequency. In order to allow a high operating frequency, the actuators are driven in the “one-node mode”, which is the frequency of the second eigenmode of a bending beam. This requires a small layer thickness of the actuators, which makes mechanical stability of the structure sensitive towards mechanical damage and constant load. At the same time, the overall system exhibits a parasitic and, in operation, undesired mode which is very close to the “one-node mode”, making operation of the device presented in EP 2233 961 A1 difficult.
FIG. 12 shows a figure from EP 2233 961 A1.
U.S. Pat. No. 6,198,565 B1 presents one way of implementing micromirrors operated in resonance, using which large deflections, high resonant frequencies and operating modes which are clearly separated from other modes can be achieved. However, it is of disadvantage with this solution that the springs connecting the micromirror to the actuators are provided with high mechanical loads, with the result that high levels of material stress are already reached with moderate mirror deflections, causing the material of the springs to fail, so that the springs will break.
FIGS. 13a and 13b show pictures of such a micromirror the spring elements of which contain defects.
U. Baran et al., in their publication “High Frequency Torsional MEMS Scanner for Displays”, have achieved an optical scanning angle of the micromirror of 38.5° at a resonant frequency of 39.5 kHz using a design presented in FIG. 14.
In this design, a cascading oscillator system is constructed from several vibration frames. The vibration frames here are formed of piezoelectric actuators which, in turn, are connected to the micromirror arranged in the center and an outer frame each via broad torsion springs. This avoids material overload and at the same allows a large scanning angle and, thus, a high amplitude and a high resonant frequency. Of disadvantage with this solution are, on the one hand, increased space requirements for the setup, since the dimensions of the individual components, due to the existence of a double frame and the large width of the springs, are correspondingly large and a relatively low energy efficiency of the setup, since both ends of the piezoelectric actuators are each mounted to be movable so that the force generated by the actuators cannot be transferred completely to the micromirror or the torsion springs.
Consequently, a concept for suspending a micromirror which allows both high amplitudes and scanning angles and high resonant frequencies would be desirable.
Thus, the object of the present invention is providing a device comprising a vibratably suspended optical element such that high material stress can be avoided and a higher resonant frequency of the optical element is allowed, while at the same time allowing energy-efficient operation of the device by an optimum flux of force.